Fluid level detectors which sense the level of a liquid contained within a receptacle are well known for use in conjunction with automotive engines. Historically, such detectors have been made in the form of float operated switches involving moving parts which are subject to friction and wear. Other devices utilize an electrical probe to detect fluid levels by measuring the conductivity of the coolant. However, these devices require complicated current amplifying systems because there is often an insufficient amount of current passing through the electrodes to power an indicator lamp. In either of these systems, the measuring sensor is located in the fluid where contaminants are likely to collect on the sensor and disturb the fluid level measurement. Furthermore, the devices in the prior art, as described, are categorized as "intrusive" in that they require an opening into the receptacle containing the fluid. This creates an additional potential for fluid leaks as well as potential deterioration of the sensing devices.
Due to the problems discussed above, it has become desirable to employ non-intrusive means to sense fluid levels contained in receptacles. These non-intrusive systems typically involve wave propagation techniques which are implemented through a transmitter/receiver system. Typically, these systems involve the transmission of an ultrasonic signal from a transmitting transducer through a fluid to a receiving transducer. Such ultrasonic transmission systems require a liquid transmission medium in order to carry the ultrasonic signal from the transmitting transducer to the receiving transducer. Lack of signal at the receiving transducer relates to a lack of liquid transmission medium, activating a no fluid present indication. However, a failure in the transducer pair or in their respective electronic connections results in a lack of signal from the receiving transducer which, in turn, activates the normal failure mode thereby falsely indicating a dry or no fluid present state.
The increasing importance of monitoring fluid levels in automotive and other applications is creating a need for more reliable non-intrusive fluid level sensors. It is, therefore, important that a fluid level detection system be developed which can provide reliable data and which does not require contact with the fluid being measured.